A U.S. B-52 bomber was sent near disputed islands in the South China Sea and another circumnavigated Japan, conducting joint military exercises with the Air Self-Defense Force, the U.S. Pacific Air Forces said Wednesday.

Monday's mission in the contested South China Sea was the first reported flight in the area by a B-52 since November.

The Pacific Air Forces (PACAF) said that the two B-52s had taken off from Andersen Air Force Base on the U.S. island territory of Guam, and participated in "routine training missions."

"One bomber conducted training in the vicinity of the South China Sea before returning to Guam, while the other conducted training in the vicinity of Japan in coordination with the U.S. Navy and alongside our Japanese air force counterparts before returning to Guam," the PACAF said in a statement.

Aircraft Spots, a Twitter account tracking movements of military aircraft, showed one B-52 as having flown near Scarborough Shoal, a disputed uninhabited reef that Beijing calls Huangyan Island.

The shoal, which is also claimed by Taipei and Manila and sits just 230 km (140 miles) from the Philippine coast, has long been a subject of speculation amid Beijing's massive land-reclamation projects in the South China Sea. Some experts believe China may seek to fortify the shoal as part of a bid to cement control of the strategic waterway.

The B-52 aircraft involved in the mission were part of the U.S. Air Force's "continuous bomber presence" based in Guam. Since 2004, the U.S. has rotated B-1, B-52 and B-2 long-range bombers out of Guam to conduct training missions in Asia.

Akin to the U.S. Navy's so-called freedom of navigation operations, in which it has sailed warships near disputed islands claimed by China in the South China Sea, the air force missions are intended to assert that the area is international airspace as well.

Beijing has built up a series of military outposts in the South China Sea, which includes vital sea lanes through which about $3 trillion in global trade passes each year.

Washington and Beijing have frequently jousted over the militarization of the South China Sea, where China, Taiwan, Vietnam, Malaysia, Brunei and the Philippines all have competing claims.

The U.S. does not maintain any claims there, but says the operations are conducted globally with the aim of promoting freedom of navigation.

China effectively seized Scarborough Shoal — a prime fishing spot — from the Philippines in 2012 after a tense standoff. In the wake of this, and a 2016 ruling by the Permanent Court of Arbitration that invalidated China's claim of sovereignty over much of the South China Sea, the coral outcrop has become synonymous with the regional power struggle in the waterway.

In January last year, the U.S. sent a guided-missile destroyer to within 12 nautical miles (22 km) of the shoal, stoking anger from China.

U.S. Indo-Pacific commander Adm. Philip Davidson said in testimony before the Senate Armed Services Committee last month that China also continues to intimidate Filipino fishermen in the area of Scarborough Shoal.

"Chinese Coast Guard vessels now fall under the command of the Central Military Commission and regularly harass and intimidate fishing vessels from our treaty ally, the Philippines, operating near Scarborough reef, as well as the fishing fleets of other regional nations," Davidson said.

A separate posting by the Aircraft Spots Twitter account showed that the exercise took one of the bombers over the East China Sea and the Sea of Japan, with the aircraft flying around Japan.

The U.S. Air Force sent two B-52s over the East China Sea in January for "routine training" near Okinawa Prefecture.

The U.S. and Japanese forces regularly conduct exercises in the East China Sea — home to the Japanese-controlled Senkaku Islands — and Beijing, which also claims the islets and calls them the Diaoyu, often dispatches government ships and aircraft to the area surrounding them.

In November 2013, China declared an air defense identification zone, in which aircraft are supposed to identify themselves to Chinese authorities, in the East China Sea. The United States and Japan have refused to recognize the ADIZ, and many observers have viewed it as an attempt by China to bolster its claims over disputed territories, like the uninhabited Senkakus.

Beijing said in 2017 that Washington should respect the ADIZ after Chinese officials warned a U.S. bomber that it was illegally flying inside the East China Sea zone. The Pentagon rejected the Chinese call and said it would continue its flight operations in the region.

The United States is obligated to defend aggression against territories under Japanese administration under Article 5 of the U.S.-Japan Treaty of Mutual Cooperation and Security, and top U.S. officials have said this extends to the Senkakus.

Training missions such as Monday's have appeared to gain more publicity amid protracted military and trade tensions between Washington and Beijing.